(function (root) {

    // Use polyfill for setImmediate for performance gains
    var asap = (typeof setImmediate === 'function' && setImmediate) ||
        function (fn) {
            setTimeout(fn, 1);
        };

    // Polyfill for Function.prototype.bind
    function bind (fn, thisArg) {
        return function () {
            fn.apply(thisArg, arguments);
        }
    }

    var isArray = Array.isArray || function (value) {
            return Object.prototype.toString.call(value) === "[object Array]"
        };

    function Promise (fn) {
        if (typeof this !== 'object') throw new TypeError('Promises must be constructed via new');
        if (typeof fn !== 'function') throw new TypeError('not a function');
        this._state     = null;
        this._value     = null;
        this._deferreds = []

        doResolve(fn, bind(resolve, this), bind(reject, this))
    }

    function handle (deferred) {
        var me = this;
        if (this._state === null) {
            this._deferreds.push(deferred);
            return
        }
        asap(function () {
            var cb = me._state ? deferred.onFulfilled : deferred.onRejected
            if (cb === null) {
                (me._state ? deferred.resolve : deferred.reject)(me._value);
                return;
            }
            var ret;
            try {
                ret = cb(me._value);
            }
            catch (e) {
                deferred.reject(e);
                return;
            }
            deferred.resolve(ret);
        })
    }

    function resolve (newValue) {
        try { //Promise Resolution Procedure: https://github.com/promises-aplus/promises-spec#the-promise-resolution-procedure
            if (newValue === this) throw new TypeError('A promise cannot be resolved with itself.');
            if (newValue && (typeof newValue === 'object' || typeof newValue === 'function')) {
                var then = newValue.then;
                if (typeof then === 'function') {
                    doResolve(bind(then, newValue), bind(resolve, this), bind(reject, this));
                    return;
                }
            }
            this._state = true;
            this._value = newValue;
            finale.call(this);
        } catch (e) {
            reject.call(this, e);
        }
    }

    function reject (newValue) {
        this._state = false;
        this._value = newValue;
        finale.call(this);
    }

    function finale () {
        for (var i = 0, len = this._deferreds.length; i < len; i++) {
            handle.call(this, this._deferreds[i]);
        }
        this._deferreds = null;
    }

    function Handler (onFulfilled, onRejected, resolve, reject) {
        this.onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : null;
        this.onRejected  = typeof onRejected === 'function' ? onRejected : null;
        this.resolve     = resolve;
        this.reject      = reject;
    }

    /**
     * Take a potentially misbehaving resolver function and make sure
     * onFulfilled and onRejected are only called once.
     *
     * Makes no guarantees about asynchrony.
     */
    function doResolve (fn, onFulfilled, onRejected) {
        var done = false;
        try {
            fn(function (value) {
                if (done) return;
                done = true;
                onFulfilled(value);
            }, function (reason) {
                if (done) return;
                done = true;
                onRejected(reason);
            })
        } catch (ex) {
            if (done) return;
            done = true;
            onRejected(ex);
        }
    }

    Promise.prototype['catch'] = function (onRejected) {
        return this.then(null, onRejected);
    };

    Promise.prototype.then = function (onFulfilled, onRejected) {
        var me = this;
        return new Promise(function (resolve, reject) {
            handle.call(me, new Handler(onFulfilled, onRejected, resolve, reject));
        })
    };

    Promise.all = function () {
        var args = Array.prototype.slice.call(arguments.length === 1 && isArray(arguments[0]) ? arguments[0] : arguments);

        return new Promise(function (resolve, reject) {
            if (args.length === 0) return resolve([]);
            var remaining = args.length;

            function res (i, val) {
                try {
                    if (val && (typeof val === 'object' || typeof val === 'function')) {
                        var then = val.then;
                        if (typeof then === 'function') {
                            then.call(val, function (val) {
                                res(i, val)
                            }, reject);
                            return;
                        }
                    }
                    args[i] = val;
                    if (--remaining === 0) {
                        resolve(args);
                    }
                } catch (ex) {
                    reject(ex);
                }
            }

            for (var i = 0; i < args.length; i++) {
                res(i, args[i]);
            }
        });
    };

    Promise.resolve = function (value) {
        if (value && typeof value === 'object' && value.constructor === Promise) {
            return value;
        }

        return new Promise(function (resolve) {
            resolve(value);
        });
    };

    Promise.reject = function (value) {
        return new Promise(function (resolve, reject) {
            reject(value);
        });
    };

    Promise.race = function (values) {
        return new Promise(function (resolve, reject) {
            for (var i = 0, len = values.length; i < len; i++) {
                values[i].then(resolve, reject);
            }
        });
    };

    /**
     * Set the immediate function to execute callbacks
     * @param fn {function} Function to execute
     * @private
     */
    Promise._setImmediateFn = function _setImmediateFn (fn) {
        asap = fn;
    };


    Promise.prototype.always = function (callback) {
        var constructor = this.constructor;

        return this.then(function (value) {
            return constructor.resolve(callback()).then(function () {
                return value;
            });
        }, function (reason) {
            return constructor.resolve(callback()).then(function () {
                throw reason;
            });
        });
    };

    if (typeof module !== 'undefined' && module.exports) {
        module.exports = Promise;
    } else if (!root.Promise) {
        root.Promise = Promise;
    }

})(this);